Effect of ovarian hormones on duration of DNA synthesis in cells of the C3H mouse mammary gland

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As measured with the double labeling method with 3H-thymidine and 14C-thymidine in single intact C3H mice about 6 months old, the duration of DNA synthesis in alveolar cells of the mammary gland varies between 14.8 and 27.6 hr, with 20.1 hr as the average. With the same method, in mice ovariectomized and treated for 3–4 days with 1 μg 17-β-estradiol plus 1 mg progesterone per day, duration of DNA synthesis in the same cells varies between 9.5 and 11.9 hr, with an average of 10.7 hr. These results demonstrate that hormones speed up DNA synthesis in the target cells considerably, as well as greatly decrease variation. Two possible hypotheses for the stimulatory effect of hormones are proposed; the first considering an actual influence on the synthetic pathway with following increase of the reaction rate, and the second considering a possible synchronizing effect on chromosome duplication. The large variation of synthetic time in intact mice is ascribed to the variable ovarian secretion. Cyclic hormonal changes are abolished in animals ovariectomized and treated steadily with hormones, with ensuing low variation of the synthetic time. Very large doses of hormones, 10 μg of 17-β-estradiol plus 10 mg of progesterone per day for 3–4 days, or prolonged treatment, 2–3 weeks, with hormones decrease duration of DNA synthesis only to the same extent that the previously reported, less massive, treatments do. The results suggest that a minimum duration of DNA synthesis exists and was reached under any of the hormonal treatments given in this study. As measured by the “wave of labeled mitoses” method, a definite, significant difference was found between minimum duration of DNA synthesis in alveolar cells and cells from buds of 10-week old mice from one side, both having an 8.8 hr synthetic time, and duct cells from the same glands on the other side, which showed a 12.2 hr synthetic time. It was found that both 17-β-estradiol and progesterone, applied independently or in combination, are capable of inducing the same minimum in the same cell type. These results suggest that the minimum duration of DNA synthesis can be a characteristic of the cell, possibly representing the peculiar sequence and spacing of chromosome replication in the particular cell type. Lastly, the results are considered in the context of control of cell proliferation. It is concluded that in the cells of the mammary gland DNA synthesis is not an unadjustable process of the generative cycle, but varies with the proliferation rate. At least for the cells of the mammary gland the hypothesis of absence of correlation between duration of the DNA synthetic phase and generation time appears to be disproved.